pass if the incoming value equals the depth/stencil buffer value.
pass if the incoming value is greater than the depth/stencil buffer value.
pass if the incoming value is greater than or equal to the depth/stencil buffer value.
pass if the incoming value is less than the depth/stencil buffer value.
pass if the incoming value is less than or equal to the depth/stencil buffer value.
never pass.
pass if the incoming value is not equal to the depth/stencil buffer value.
off
always pass.